This invention is in the field of chemical processes; more specifically, this invention relates to methods for oxidizing organic compounds whose molecular structures include at least one carbon-phosphorous bond. The process is especially useful in the oxidation of organic phosphinates to inorganic orthophosphates.
In a broad sense, organophosphorous compounds contain at least one phosphorous atom chemically bonded directly or indirectly to a carbon skeleton. The phosphorous atom may be a member of an organic chain or ring system in which phosphorous is bonded directly to carbon, or phosphorous may be the central atom in a functional group which in turn is bonded to a carbon-containing chain or ring. Such functional groups commonly have one or more oxygen atoms or hydroxyl (xe2x80x94OH) groups surrounding the phosphorous atom.
Examples of organophosphorous compounds in which phosphorous is bonded directly to carbon include: phosphines R3P, phosphine oxides R3PO, phosphonic acids RP(O)(OH)2, phosphonous acids RPH(O)(OH), phosphinous acids R2POH, phosphoranes R3Pxe2x95x90CH2, and biphosphines R2Pxe2x80x94PR2. Examples of organophosphorous compounds in which phosphorous is bonded both directly and indirectly to carbon include phosphonates RP(O)(ORxe2x80x2)2, phosphonites RPH(O)(ORxe2x80x2), phosphinites R2P(ORxe2x80x2), phosphonamides RP(O)(NRxe2x80x2Rxe2x80x3)2, and phosphinamides R2P(NRxe2x80x2). The alkyl phosphonates and phosphinates are related to phosphonic acid HP(O)(OH)2 and phosphinic acid H2P(O)OH, respectively. In all of these examples the R, Rxe2x80x2 and Rxe2x80x3 moieties can be the same or different and any combination of alkyl or aromatic groups. Further examples of the large number of these compounds can be found in the seminal textbook xe2x80x9cOrganophosphorous Compoundsxe2x80x9d by G. Kosolapoff, John Wiley and Sons, New York, N.Y., 1950.
The organophosphorous compounds within the scope of this invention all contain phosphorous directly bonded to carbon. The compounds additionally may include phosphorous indirectly connected to carbon, but the structural feature of the organophosphorous compounds central to the process of this invention is:
Pxe2x80x94Cxe2x80x83xe2x80x83(I)
Although the process of this invention is of general utility, it is especially useful when applied to organophosphorous compounds represented generally by chemical structure II: 
The compounds of formula II are called xe2x80x9cphosphinates.xe2x80x9d
Within the set of the phosphinates of structure II is a subset, a number of which are chemical warfare agents; more specifically, the subset includes some extremely toxic cholinesterase inhibitors, i.e., nerve agents. This subset is included in the compounds represented by formula III: 
in which R1 is selected from hydrogen and alkyl, R2 is independently selected from alkyl, and Y is a leaving group.
Destruction of chemical warfare agents, including nerve agents within the scope of formula III, is the subject of PCT Application WO 97/18858, published May 29, 1997 and counterpart U.S. Pat. 5,598,691, dated Dec. 7, 1999, either or both of which are referred to as xe2x80x9cthe earlier patentxe2x80x9d hereinafter and are incorporated herein by reference. Included in the disclosure of the earlier patent is a preferred process for destroying chemical warfare agents by subjecting them to a xe2x80x9cdissolving metal reduction.xe2x80x9d The reduction involves creating a reaction mixture prepared from raw materials which include nitrogenous base, e.g., anhydrous liquid ammonia, at least one chemical warfare agent, e.g., a nerve agent of formula III, and an active metal, such as sodium. The dissolving metal reaction generates solvated electrons, a highly active reducing agent. The reduction reaction results in a product which includes, among other substances, the phosphinate salts represented by formula IV: 
in which R1 and R2 are the same as in formula III, and Z is a cation of charge n. Noteworthy is the fact that the salts of formula IV still contain phosphorous-carbon bonds Pxe2x80x94R2.
The earlier patent suggests oxidation of the product from reduction of the chemical warfare agent in order to simplify its disposal as waste. However, the suggested oxidation with agents, such as hydrogen peroxide may leave some of the Pxe2x80x94R2 phosphorous-carbon bonds intact. The continued presence of phosphorous-carbon bonds raises toxicity issues and presents additional waste disposal problems.
Thus, it would be beneficial to provide a method for breaking the carbon-phosphorous bonds found in the organophosphorous compounds of the aforesaid formulae in order to provide a new synthetic route to useful materials and also to reduce the toxicity of the organophosphorous compounds of the aforesaid formulae and render them more environmentally acceptable.
Therefore, it is an objective of this invention to provide a method for attaining these ends starting with organophosphorous compounds having the structural feature of formula I. It is another objective to provide a process which is especially applicable to break the carbon-phosphorous bonds in organophosphorous compounds within the scope of formula II. Yet another objective is to provide a unique method for destroying, i.e, breaking the carbon-phosphorous bond in the phosphinates of formula III, including destruction of those compounds of formula III which are chemical warfare agents. It is still another objective to provide a method for oxidizing compounds of formula IV, especially as a follow-up to the dissolving metal reduction of the chemical warfare agents within the scope of formula III, thereby decreasing the toxicity of the reduction products and simplifying their disposal as waste.
Accordingly, in attaining the aforesaid objectives this invention provides a process for converting organophosphorous compounds of formulae II, III, and IV, all of which contain the structural feature of formula I, into relatively benign organic compounds and inorganic salts which often are soluble in water and are relatively safe, environmentally.
In order to accomplish these objectives, it is preferred to treat the organophosphorous compounds with an oxidizing agent comprising a peroxysulfate of formula V,
xe2x80x83M2SxOyxe2x80x83xe2x80x83(V)
in which M is a monovalent cation, x is 1 or 2, y is 5 when x is 1, and y is 8 when x is 2; M2SO5 is a peroxymonosulfate, while M2S2O8 is a peroxydisulfate. The process is preferably carried out in the presence of water, and in addition, should be conducted in an alkaline pH range. This is in contrast to the methods of John F. Cooper, et al, described in a paper entitled xe2x80x9cDestruction of VX by Aqueous-Phase Oxidation Using Peroxydisulfate (Direct Chemical Oxidation)xe2x80x9d presented at the Workshop on Advances in Alternative Demilitarization Technologies, Reston, Va., Sep. 25-27, 1995. The oxidation processes of Cooper et al were carried out using acidified ammonium peroxydisufate with pHs adjusted down to 1.5. Unlike the methods of Cooper et al, these inventors found that although some reaction occurs in the acid pH range, cleavage of the carbon-phosphorus bond is greatly facilitated when performed under alkaline conditions, i.e., pH greater than 7, and at elevated temperatures. The addition of base, such as sodium hydroxide also allows lesser amounts of the peroxysulfate to be used, making it more economic.
The chemistry of peroxysulfate oxidation/reduction, including kinetics and mechanism, has been described in I. M. Kolthoff and I. K. Miller, J. Am. Chem. Soc., 73, 3055 (1951) and in L. S. Levitt, Can. J. Chem., 31, 918 (1963). The oxidation of methylphosphonic acid, CH3P(O)(OH)2, using ozone/oxygen at pH 8-8.5 in aqueous sodium bicarbonate to yield phosphoric acid has been reported; see V. V. Smirnov, et al., Zh. Obshch. Khim., 37(12) 2783-4 (1967); C.A. 68:113762x. Related disclosures are found in Zh. Obshch. Khim., 38(5) 1197 (1968); C.A. 69 77373f and Zh. Obshch. Khim., 39(4) 932 (1969); C.A. 71 56020q. The oxidation with aqueous HNO3/KMnO4 of a series of compounds RPO3H2 in which R is alkyl is described in Dokl. Akad. Nauk. SSSR, 167(6) 1303-5 (1966); C.A. 65 3902b.
The result of the process is to break at least one carbon-phosphorous bond, typically yielding a phosphorous-containing acid or a salt thereof, depending upon the nature of the cation M, pH and the substituents bonded to phosphorous. For example, in the general case and employing a peroxydisulfate, the structural feature of formula I undergoes the reaction shown in chemical equation VI:
Pxe2x80x94C+xc2xdM2S2O8+H2O+MOHxe2x86x92Pxe2x80x94OM+HOxe2x80x94C+MHSO4xe2x80x83xe2x80x83(IV)
While specifically illustrated with the organophosphorous structure of formula I, those skilled in the art will be able to extend the process readily to other organophosphorous compounds, including, but not limited to, the compounds of formulae II, III and IV. For example, oxidation of a phosphinate salt of formula IV according to the process of this invention leads to orthophosphoric acid or a salt thereof depending upon the specifics of the reactants employed as shown in chemical equation VII (when Zxe2x95x90M=Na+). 
Whereas organophosphorous compounds with the structural features of formulae I-IV, as well as mixtures thereof, can be oxidized using the process of this invention, those skilled in the art will understand that the path taken and the results achieved may vary, depending upon a number of factors. For example, the organophosphorous compound desired to be oxidized may contain other structural features which will interfere with the peroxysulfate oxidizing agent, such as other oxidizable groups. This can lead to side reactions. Peroxysulfate salts are generally employed in aqueous media, but if the organophosphorous compound is not soluble in water, the course of the perhaps heterogeneous reaction becomes uncertain. In addition, while an important feature of the invention includes conducting the oxidation reaction at an alkaline pH, i.e. pH greater than 7, and more specifically, at a pH in the range of about 8 to about 12, or more, this may, in some circumstances, adversely affect other features on the molecule, e.g., may hydrolyze ester functionality.
Thus, application of the process of this invention requires the knowledge and skill possessed by trained chemists or chemical engineers after careful consideration of the nature of the organophosphorous reactant.
In applying the process of this invention to break a carbon-phosphorous bond, a solution of peroxysulfate of formula V in water is advantageously employed. The cationic moiety M in formula V can be, e.g., an alkali metal, such as Na+ or K+, as well as NH4+, e.g., sodium peroxydisulfate (or sodium persulfate), ammonium peroxymonosulfate, all of which are available through ordinary channels of commerce. Mixtures of peroxysulfates, etc., can also be utilized.
As previously mentioned, in addition to the peroxysulfate and water, the reaction mixture contains base. Although a wide variety of bases can be used, good results are obtained, and less expense incurred by employing strong base, such as, for example, the hydroxides of alkali and alkaline earth metals or mixtures thereof. Specific examples include the hydroxides of lithium, sodium, potassium, calcium and magnesium; among these, sodium hydroxide is readily available at reasonable cost, and so is preferred. Base is employed in quantities sufficient to achieve the alkaline pH ranges discussed hereinabove.
The reaction mixture can be created using various orders of addition. In general, the reaction mixture preferably is created by heating a mixture of water, an organophosphorous compound containing the structural feature of formula I, e.g., an organophosphinate of formula II, III, IV, or a mixture thereof, and base in a stirred vessel under atmospheric or higher pressure. The vessel is generally equipped with a condenser, optionally topped with a gas collection device. Sufficient aqueous peroxysulfate of formula V is added slowly to the mixture at a rate so as to maintain a temperature below about 100xc2x0 C.
Typically, for each 1.0 part by weight organophosphorous compound, about 5-200 parts by weight water, about 5-100 parts peroxysulfate by weight, and up to about 50 parts of base, such as sodium hydroxide, by weight will be combined to create the reaction mixture.
The process of this invention can be applied to compounds containing the structure of formula II, including that subset of organophosphinates represented by formula III. The products of the reaction generally include a phosphate, also called an orthophosphate, derived from phosphoric acid, H3PO4.
In formula III, Y is an atomic grouping which is energetically stabile as an anion, the more preferred leaving groups being those which are most readily displaced from carbon in nucleophilic substitutions and, as anions, have the greatest stability. Although a host of such leaving groups are well known, it is preferred that the leaving group Y be selected from halogen, nitrile (xe2x80x94CN), and sulfide (xe2x80x94Sxe2x80x94). These are the groups Y present in the organophosphinates of greatest interest as nerve agents. Among the halogens, it is most preferred that Y be fluorine, chlorine or bromine, fluorine being especially effective in the most readily available nerve agents, such as xe2x80x9cSomanxe2x80x9d or xe2x80x9cGD.xe2x80x9d
R1 and R2 in formulae II and III can both be alkyl, and when they are, they can be the same or different; they can be selected independently from alkyl, preferably lower alkyl, i.e., C1-C6, straight chain, branched or cyclic, e.g., methyl, ethyl, propyl, iso-propyl, iso-butyl, tert-butyl, cyclohexyl, or trimethylpropyl. R1 in the most widely distributed nerve agents is methyl, ethyl or 1,2,2-trimethylpropyl.
Specific agents within the scope of formula III include, e.g., xe2x80x9cSarin,xe2x80x9d or xe2x80x9cGB,xe2x80x9d or methylphosphonofluoridic acid 1-methyl ethyl ester, or isopropyl methyl phosphonofluoridate; xe2x80x9cSoman,xe2x80x9d or xe2x80x9cGD,xe2x80x9d or methylphosphonofluoric acid 1,2,2-trimethylpropyl ester, or pinacolyl methyl phosphonofluoridate; and xe2x80x9cVX,xe2x80x9d if or methylphosphonothioic acid S-[2-[bis (1-methyl ethyl)amino]ethyl]ethyl ester, or ethyl S-2-diisopropyl aminoethyl methylphosphoro-thioate.
In carrying out the oxidation of this invention with an organophosphinate of formula II or III, reaction mixtures preferably are prepared utilizing, for each 1.0 g phosphinate, about 5-200 g water, about 5-100 g peroxysulfate, e.g., sodium peroxydisulfate, and up to about 50 g base, e.g., sodium hydroxide. The preferred procedure involves preparing a solution of the base in water with stirring and control of the temperature so as not to exceed about 85xc2x0 C. The organophosphinate is then added to the basic solution at a rate such that the temperature of the mixture does not exceed about 85xc2x0 C. Finally, the peroxysulfate is added, preferably as an aqueous solution, so as to maintain the temperature of the mixture at about 85xc2x0 C.-95xc2x0 C. Crystalline peroxydisulfate containing 32.5% water is available as a commercial product.
If the phosphinate starting material is a chemical warfare agent, reference should be made to the earlier patent for general handling procedures. Upon completion of the reaction, especially if the phosphinate reactant is a nerve agent, the product can be analyzed for cholinesterase inhibition should residual biological activity be of interest; see. e.g., M. Waters, xe2x80x9cLaboratory Methods for Evaluating Protective Clothing Systems Against Chemical Agents,xe2x80x9d CRDC-SP-84010, U.S. Army Armament, Munitions and Chemical Command, Aberdeen Proving Ground, Md. 21010 USA, June 1984. The product, substantially free of carbon-phosphorous bonds, can be retained for further reaction or safely disposed of.
Oxidation of phosphinate salts of formula IV can be carried out by the process of this invention regardless of their source. In formula IV, Z+n, a cation of charge n, can be selected from NH4+; alkali metals cations, such as Li+, Na+ or K+; alkaline earth metal cations, such as Ca+2, Ba+2 or Mg+2; as well as Al+3, Ti+4, and so forth. In many cases Z+n will be an alkali metal cation, such as sodium, for reasons of cost and availability.
Should the salt of formula IV be available in solid form, a weighed amount can be dissolved in a measured quantity of water. If the salt is contained in anhydrous liquid ammonia or other nitrogenous base, e.g., as the product from the dissolving metal reduction of a phosphinate of formula III, water in measured amount can be added cautiously to the mixture in order to decompose any residual active metal. This will yield an aqueous solution from which the ammonia or other nitrogenous base can be substantially removed, e.g., by evaporation.
The amount of phosphinate salt in the aqueous solution, if unknown, can be determined by isolating and weighing the salt, or spectroscopic techniques, optionally with reference to independently prepared standards, can be employed. Alternatively, if previous experience permits, one can assume that a certain fraction (generally 100%) of the starting organophosphorous compound has been converted to phosphinate salt. In these ways the weight of solution containing 1.0 g of the phosphinate salt reactant can be ascertained.
Reaction mixtures are prepared per about 1.0 g phosphinate salt of formula IV, about 5-200 g water, about 5-100 g peroxydisulfate, e.g., sodium peroxydisulfate, and sodium hydroxide to about 50 g, and more particularly, from about 4 to about 50 g. The orthophosphate salt or acid produced can be quantified by using a standard molybdate calorimetric test or by ion chromatography. Preferably, for 1.0 g phosphinate salt, about 25-0 g water, about 5-50 g peroxydisulfate, and about 4-40 g base are used to prepare the reaction mixture. Most preferably, about 10-15 g sodium peroxydisulfate, about 6-30 g sodium hydroxide, and about 40-100 g water are used for each 1.0 g of phosphinate salt. It will be evident that the amounts of the reactants can be scaled up or down, depending upon the size of the equipment and the identities of the specific reactants.
The result of carrying out the process of this invention is generally influenced more by the amount of the peroxysulfate of formula V used than by any other single variable, while varying the amount of water within the stated limits usually has minimal effect. Under optimum conditions, the process results in a quantitative yield of orthophosphate or orthophosphoric acid, depending upon the pH.